The invention relates to a free space optical interconnect system and a method of operation thereof, in particular to the system and method providing tolerance to misalignments.
Free space optical interconnect systems have long been proposed to deliver fast, highly parallel data transfer. These systems have the potential to obviate limitations of electrical interconnects, which are not capable of supporting data throughputs beyond a capacity of several hundred Gb/s, and to increase the capacity up to the Terabit/s range. Thus free space interconnect systems are promising and attractive alternatives for various telecommunication and computing applications.
However, the most important challenge preventing the current acceptance of free space interconnect systems is alignment. Two issues are of concern: the precision to which it is possible to align the system, and the precision to which it is necessary to maintain this alignment during operation. For practical applications it is necessary to establish and maintain alignment of circuit boards carrying transmitters and receivers, which may comprise an array of pixels, to within 10""s of microns over a distance of meters. Such a system requires extremely expensive highly precision optomechanics, and to date has been implemented only in a controlled laboratory environment. In real product usage, when vibrations, temperature fluctuations and temperature gradients are encountered, the optical links move out of alignment and data is not correctly transferred.
Therefore, the goal of providing some alignment tolerance for optical links is to ensure the correct operation of all of the pixels on each array at the highest possible speed. Correct operation is defined as the correct reception of a logic 1 or logic 0 signal. Once the laser power, the receiver sensitivity and the detector area have been defined, the probability of correct reception of the logic bits is mainly a function of optical beam misalignment. Misalignment mechanisms can be due solely to mechanical movements, but in practice, optical effects can also contribute. Six degrees of freedom of the mechanical movements: translation in x, y, and z (xcex94x, xcex94y, xcex94z) and rotation about the x, y, and z axes (xcex8x, xcex8y, xcex8z), where x and y axes define the plane of an optical module in its nominal alignment position, with z axis being perpendicular to this plane, result in a number of optical effects. These include an image shift (xcex94x, xcex94y), image rotation (xcex8z), defocus (xcex94z) and image tilt (xcex8x, xcex8y). Image shift and rotation are basically lateral translation effects, and defocus and image tilt introduce defocus effects. Contributors to the overall lateral misalignment effects include:
mechanical misalignment in x and y;
mechanical rotation about the z axis;
mismatches in focal lengths;
wavelength shifts and laser mode-hops caused by temperature fluctuations and resulting in beam deflections introduced by diffractive elements;
distortions of the image of an array of sources by the interconnect lens system, and
telecentricity, when defocus, in addition to increasing spot size, introduces lateral misalignments in nontelecentric systems.
Contributors to the overall defocus effects include:
source array tilt;
image tilt;
curvature of the plane of best focus;
mechanical tilt about x and y axes;
misalignment along z axis.
Numerous attempts have been made to increase alignment tolerance for optical interconnect systems which may be categorized as passive, active, or dynamic strategies.
However, passive alignment of dense, high speed free space optical interconnects for distances of more than 1 cm require mechanical support structures that are too expensive, difficult to align, and insufficiently stable for commercial applications, see, e.g., xe2x80x9cOptoelectronic ATM switch employing hybrid silicon (MOS/GaAs) FET-SEEDSxe2x80x9d, A. L. Lentine et al., SPIE Proceeding, vol. 2692, pages 110-108, 1996; and xe2x80x9cOptical bus implementation system using Selfoc lensesxe2x80x9d, K. Namanaka, Optics Letters, Vol. 16, No. 16, pp. 1222-1224, August 1991. Passive alignment is done before any devices are powered up. This alignment technique is used in almost all electrical connectors, and most optical fiber connectors are passive. Recently, solder bump techniques have been applied to certain free space optical interconnect components, and preliminary reports indicate the potential for submicron alignment in all 6 degrees of freedom over a scale of up to 1 cm, J. W. Parker xe2x80x9cOptical Interconnection for Advanced Processor Systems: A Review of the ESPRIT II OLIVES Programxe2x80x9d, L. Lightwave Technology 9 (12), 1764-1773, 1991.
Active alignment requires some feedback about the quality of the alignment. Usually the feedback is achieved by illuminating the system and monitoring the alignment either visually or by measuring a photocurrent in the detectors. Real-time active alignment is necessary if the alignment tolerances are tight or the system stability is poor so that the system will not remain aligned for a reasonable length of time. In this case, the feedback and alignment actuators must be integrated into the system to ensure permanent alignment. For example, CANON manufacturer uses image recognition and active beam-steering using a liquid filled variable angle prism in a single channel 155 Mb/s link product, which currently costs $100K. The product uses built in viewing cameras and optical pattern recognition techniques to define the system alignment, the complexity and cost of such a system clearly limiting widespread application. Alternatively, NTT has a system using actively controlled variable angle liquid filled prisms for board to board parallel free space optical interconnect, see. e.g. xe2x80x9cOptical beam direction compensating system for board-to-board free space optical interconnection in high-capacity ATM switchxe2x80x9d, K. Hirabayashi et al., Journal of Lightwave Technology, Vol. 15, No. 5, May 1997. Cost, size, environmental ruggedness and reliability of these systems remain concerns.
Additionally, to develop both a marketable and reliable system, devices have to be packaged in a useful and reliable manner. For large systems including cumbersome and bulky mechanical parts providing alignment, this could involve a significant amount of physical space just to house all the individual components.
Recently, a proposal for avoiding high precision mechanics in free space interconnect systems by use of redundant arrays of detectors has been put forward by F. A. P. Tooley in IEEE Journal of Selected Topics in Quantum Electronics April 1996, vol. 2, No. 1, pp. 3-13 and in Digest, IEEE Summer Topical Meetings, Aug. 5-9 1996, p. 55-56. This system increases tolerance to misalignment by providing an array of detectors in place of a single detector and electrically re-routing the misaligned optical data to the correct channel, or, alternatively, by replicating the signal a number of times. The overhead associated with increasing the alignment tolerance requires a control and router circuit, which adds electrical power consumption.
In patent application Ser. No. 09/150,242 to Dominic Goodwill it has been proposed to arrange redundant elements into redundant clusters, the number of elements in each cluster being sufficient to accommodate the number of data channels to be transmitted. The system also includes means for identifying a misalignment between the transmitter and the receiver, and means for re-routing data from the cluster which is misaligned to the redundant cluster which thus re-directs data to/from the correct physical location.
Unfortunately, there is a drawback associated with the use of redundant elements in optical interconnect systems. Re-routing of data between the redundant elements or clusters requires time for hunting for an appropriate available element or cluster which would provide reliable data transmission through the system. While hunting, some of the data transmitted is inevitably lost. Accordingly, the faster the hunting and re-routing, the more reliable the optical interconnect system is. If the hunting process takes too long, it may result in losing the optical link at all which is not acceptable in many circumstances. It also limits general applications of free space optical interconnect systems.
Therefore there is a need for development of free space optical interconnect systems tolerant to misalignments and methods of operation thereof which would provide reliable data transmission through the system.
Thus, the present invention seeks to provide an optical interconnect system and method which avoids or reduces the above-mentioned problems.
Therefore, according to one aspect of the present invention there is provided a free space optical interconnect system comprising:
a transmitter and a receiver, at least one of the transmitter and the receiver comprising a plurality of elements whose number is redundant;
means for monitoring a misalignment between the transmitter and the receiver including means for determining a direction and an amplitude of the misalignment; and
means for re-routing data from the element which is misaligned to a redundant element selected along a direction associated with the direction of the misalignment so as to provide data transmission through the system, the re-routing being performed in response to a signal generated by the monitoring means.
The means for monitoring the misalignment may include means for monitoring a signal connection parameter between the transmitter and the receiver, e.g. a signal parameter at the receiver or at the transmitter. Alternatively the signal connection parameter may be a signal parameter of at least one element of at least one of the transmitter and the receiver. Conveniently, the signal connection parameter is an intensity of the data signal.
In an embodiment of the invention, means for monitoring the misalignment between the modules includes a dedicated alignment laser and a dedicated detector, with the means for determining the direction and the amplitude of the misalignment including a circuitry for measuring a position of the laser spot of the alignment laser on the dedicated detector. Alternatively, means for monitoring the misalignment may include a detector selected from the group consisting of detectors for monitoring lateral and vertical misalignments, and detectors for monitoring tilt misalignments or other known suitable detectors. Optionally, the means for monitoring the misalignment may further comprise means for providing feedback between the transmitter and the receiver regarding the misalignment, which can be conveniently selected from optical fiber, LED, electrical cable, electrical backplane or other suitable means. As an alternative to the embodiment described above, means for determining the direction and the amplitude of the misalignment may include means for measuring an intensity distribution at the receiver elements.
The elements of the transmitter and/or receiver may be arranged into a one-dimensional or two-dimensional array, or any other pattern providing the required optical transmission or collection. Alternatively, the elements of the transmitter and/or receiver may be arranged into clusters, the number of clusters being redundant and the number of elements in each cluster being sufficient to accommodate the number of data channels to be transmitted. If required, the elements may be shared by different clusters. The system may comprise one transmitter and one receiver only to provide a uni-directional interconnection. Alternatively, the system may comprise two modules, each module having one transmitter and one receiver, thus providing for a bi-directional transmission and receiving of data. It may be arranged that the receiver only has redundant elements. If required, the transmitter only may have redundant elements or redundant clusters.
Preferably, the system is implemented with optical elements, such as bulk optics (lenses, prisms, mirrors, splitters, et al.), binary optics (fanout gratings, diffractive lenses, et al.), holographic elements, and integrated optics.
Preferably, the elements of the transmitter are optical emitters or optical modulators. The emitters may be vertical cavity surface emitting lasers (VCSEL), light emitting diodes (LED) and edge emitting laser diodes or other known devices. The modulators may be modulators based on magneto-optic effect, modulators including liquid crystal devices, ferroelectric modulators, e.g. lead lanthanum zirconate titanate (PLZT) modulator, modulators including piezo-electric crystals, modulators including deformable mirrors, electro-optical semiconductor hetero-structure modulators, optical cavity modulators, or other known modulators.
The receiver of the optical interconnect system comprises at least one detector, preferably from PIN detector, metal-semiconductor-metal detector, avalanche photodiode, or other known detectors.
Preferably, the transmitter and/or receiver, or the whole system described above are integrated within a package or several packages, thus providing compactness and efficient use of space.
According to another aspect of the invention there is provided a module for free space optical interconnect system, comprising:
at least one of a transmitter and a receiver, at least one of the transmitter and the receiver comprising a plurality of elements whose number is redundant;
means for monitoring a misalignment of the module including means for determining a direction and an amplitude of the misalignment; and
means for re-routing data from the element which is misaligned to a redundant element selected along a direction associated with the direction of the misalignment so as to provide data transmission through the system, the re-routing being performed in response to a signal generated by the monitoring means.
Beneficially, the means for monitoring the misalignment of the module comprises means for monitoring a signal connection parameter at the module, e.g. an intensity of the signal. Alternatively, the means for monitoring the misalignment may comprise detectors for monitoring lateral and vertical misalignments, detectors for monitoring tilt misalignments or any other suitable known detectors. In an embodiment of the invention, means for monitoring the misalignment of the module includes a dedicated detector, with the means for determining the direction and the amplitude of the misalignment including means for measuring a position of a laser spot of a dedicated alignment laser on the dedicated detector. Alternatively, it may include means for measuring an intensity distribution, e.g. at the receiver elements. The module may include one transmitter or one receiver only for a uni-directional link. If required, it may include both the transmitter and receiver for corresponding transmitting and receiving of data in a bi-directional optical interconnect system. Preferably, the elements of the module are arranged into a one-dimensional array or two-dimensional array. Alternatively, they may be arranged so as to form a pre-determined pattern providing the required optical transmission or collection. Optionally, the redundant elements may be arranged into clusters, the number of clusters being redundant and the number of elements in each cluster being sufficient to accommodate the number of data channels to be transmitted. Accordingly, the means for re-routing comprises means for re-routing data from a cluster which is misaligned to a redundant cluster which provides data transmission through the system. If required, the elements may be shared between different clusters. Beneficially, the elements of the transmitter are optical emitters, e.g. VCSEL, SLD, LED, edge emitting laser diodes or other known emitters. Alternatively the elements of the transmitter may be optical modulators. The elements of the receiver may be selected from PIN detector, metal-semiconductor-metal detector, avalanche photodiode or other known suitable detectors. Beneficially, the module is integrated within a package.
According to yet another aspect of the invention there is provided a method of operating a free space optical interconnect system, comprising a transmitter and a receiver, at least one of the transmitter and the receiver having a plurality of elements whose number is redundant, the method comprising the steps of:
(a) monitoring a misalignment between the transmitter and the receiver, including determining a direction and an amplitude of the misalignment; and
(b) when the amplitude of the misalignment is exceeding a pre-determined threshold value, re-routing data from the element which is misaligned to a redundant element selected along a direction associated with the direction of the misalignment so as to provide data transmission through the system.
Conveniently, the step of monitoring the misalignment comprises monitoring a signal connection parameter between the transmitter and the receiver, e.g. a signal parameter at the receiver or at the transmitter. Optionally, the step of monitoring the signal connection parameter comprises monitoring a signal parameter of at least one element of at least one of the transmitter and the receiver, e.g. monitoring intensity of the data signal. In the embodiment of the invention, the step of determining the direction and the amplitude of the misalignment comprises measuring a position of a laser spot of a dedicated alignment laser on a dedicated detector. Alternatively, this step may include measuring an intensity distribution, e.g. at the receiver elements.
Free space optical interconnect systems formed using the techniques described above are more reliable compared to other existing free space interconnect systems having redundant elements. Monitoring of the misalignment between the transmitter and the receiver, determining the direction and the amplitude of the misalignment and comparing the amplitude with the threshold value allows re-routing of data to available redundant elements well in advance before the quality of data transmission deteriorates substantially and before the link is dropped or data is lost. The use of redundant elements also obviates the need of packaging which requires precise alignment and which is often expensive and bulky. The interconnect systems based on the present invention have simpler mechanical design, have no moving parts and may be implemented with lower cost mechanics. As a result, they can be manufactured more readily and at much lower cost, and providing higher reliability at the same time.